With alias you can give a type a different name:

alias PInt32 = Pointer(Int32)

ptr = PInt32.malloc(1) # : Pointer(Int32)

Every time you use an alias the compiler replaces it with the type it refers to.

Aliases are useful to avoid writing long type names, but also to be able to talk about recursive types:

Inch Rocky Iron LTT Boot Brown Clad Men's Eight Work alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Brown Work Iron Clad Boot LTT Men's Inch Rocky Eight 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Dawson American Mens Brown Trask Steer vqSz5g

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)
end